Well in all honesty I'm not sure anymore why we're doing it. But I will say that having the ability to use C++ strings will be well worth it even if that's the only tangible benefit people see. Strings which don't overflow their buffers and functions to manipulate them that make a hell of alot more sense than trying to walk arrays of characters.
STL lists are far safer to use than the LINK/UNLINK macros too, so that will be of great benefit to people. std::bitset allows for what appear to be limitless bitvectors, vs the 128bit cap on the EXT_BV hack structure, that conversion has been done on all bitvector sets. File stream IO seems to be far less picky about the way you handle them, so that will be helpful too. There are some instances where std::map comes into play and makes the data that uses them much easier to fiddle with.
In the end, it's not trying to become a convoluted mass of inhereted classes and structs, with templates out the wazoo, but merely something that leverages what works to make life easier.